Blockchain is a specific type of database.
It differs from a typical database in the way it stores information; blockchains store data in blocks that are then chained together.
As new data comes in it is entered into a fresh block. Once the block is filled with data it is chained onto the previous block, which makes the data chained together in chronological order.
Different types of information can be stored on a blockchain but the most common use so far has been as a ledger for transactions.
In Bitcoin’s case, blockchain is used in a decentralized way so that no single person or group has control—rather, all users collectively retain control.
Decentralized blockchains are immutable, which means that the data entered is irreversible. For Bitcoin, this means that transactions are permanently recorded and viewable to anyone.